1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to improvement in dimples of golf balls.
2. Description of the Related Art
Golf balls have a large number of dimples on the surface thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. By causing the turbulent flow separation, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulent flow separation promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. The reduction of drag and the enhancement of lift force are referred to as a “dimple effect”.
In a golf ball, the ratio of the sum of the areas of all dimples to the area of the phantom sphere is referred to as an occupation ratio. A golf ball with a high occupation ratio achieves a superior dimple effect. U.S. Pat. No. 5,292,132 (JP-H4-347177) discloses a golf ball with an increased occupation ratio.
The United States Golf Association (USGA) has established the rules about symmetry of golf balls. According to the rules, the trajectory during PH (poles horizontal) rotation and the trajectory during POP (poles over pole) rotation are compared with each other. The rotation axis for PH rotation extends through both poles of the golf ball, and the rotation axis for POP rotation is orthogonal to the rotation axis for PH rotation. A golf ball having a large difference between a trajectory during PH rotation and a trajectory during POP rotation, that is, inferior aerodynamic symmetry, does not conform to the rules.
Other than a shot at a teeing ground, golf players cannot decide a hitting place of a golf ball. Thus, a flight distance of a golf ball with inferior aerodynamic symmetry varies. Golf players have difficulty in landing this golf ball at an intended point.
The dimples can be arranged by using a regular polyhedron that is inscribed in the phantom sphere of a golf ball. In this arrangement method, the surface of the phantom sphere is divided into a plurality of units by division lines obtained by projecting the sides of the polyhedron on the spherical surface. The dimple pattern of one unit is developed all over the phantom sphere. According to this dimple pattern, the aerodynamic characteristic in the case where a line passing through a vertex of the regular polyhedron is a rotation axis is different from that in the case where a line passing through the center of a surface of the regular polyhedron is a rotation axis. Such a golf ball has inferior aerodynamic symmetry.
U.S. Pat. No. 4,936,587 (JP-S50-8630) discloses a golf ball having an improved dimple pattern. The surface of the golf ball is divided by an icosahedron that is inscribed in the phantom sphere thereof. Based on this division, dimples are arranged on the surface of the golf ball. According to this dimple pattern, the number of great circles that do not intersect any dimples is 1. This great circle agrees with the equator of the golf ball. The region near the equator is a unique region.
Generally, a golf ball is formed by a mold having upper and lower mold halves. The mold has a parting line. A golf ball obtained by this mold has a seam at a position along the parting line. Through this forming process, spew occurs along the seam. The spew is removed by means of cutting. By cutting the spew, the dimples near the seam are deformed. In addition, the dimples near the seam tend to be orderly arranged. The seam is located along the equator of the golf ball. The region near the equator is a unique region.
A mold having an uneven parting line has been used. A golf ball obtained by this mold has dimples on the equator thereof. The dimples on the equator contribute to eliminating the uniqueness of the region near the equator. However, the uniqueness is not sufficiently eliminated. This golf ball has insufficient aerodynamic symmetry.
U.S. Pat. No. 4,744,564 (JP-S61-284264) discloses a golf ball in which the dimples near the seam are greater in volume than the dimples near the poles. This volume difference contributes to eliminating the uniqueness of the region near the equator.
The golf ball disclosed in U.S. Pat. No. 4,744,564 eliminates, by the volume difference of dimples, the disadvantage caused by the dimple pattern. The disadvantage is eliminated not by modification of the dimple pattern. In the golf ball, the potential of the dimple pattern is sacrificed. The flight distance of the golf ball is insufficient. An object of the present invention is to provide a golf ball having excellent flight performance.